Medicare Advantage plan enrollment fell 2.8% in January, compared to the prior month, leading one analyst to call the preliminary results for the 2010 open-enrollment period “troublesome.”

Stand-alone prescription drug plans gained 70,195 members during the same period, according to the report, prepared by Mark Farrah Associates, a health plan market data firm based in Kennebunk, Maine.

Medicare Advantage plans reported enrollment of 10,971,598 for January 2010, while the stand-alone PDPs logged 17,664,256 enrollees, as of Jan. 1, the report said.

“The downward trend for Medicare Advantage is troublesome,” said Debra A. Donahue, vice president of market analytics for Mark Farrah Associates. “Though year-over-year enrollment is up 5.2%, sustaining Medicare Advantage levels with reimbursement cuts on the way has been challenging for plans nationwide.”

The report found Medicare Advantage declines in almost every state, except Minnesota, Georgia and Ohio.

The results are preliminary because it only accounts for transactions through Dec. 11, 2009, when the open-enrollment period spans mid-November to Dec. 31 each year. People who were previously enrolled in a Medicare Advantage plan can switch their coverage through March.

Mark Farrah Associates said consolidations, terminations and withdrawals from the program contributed significantly to the Medicare Advantage decline.

The analysis could not determine yet if the declines were from people either abandoning Medicare Advantage plans or returning to fee-fore-service Medicare coverage.